impedance
In an AC circuit, resistors, capacitors, and inductors all oppose the current — but capacitors and inductors do it in a way that depends on the frequency and on timing, not just on their size. Impedance is the total opposition to AC, generalising plain resistance. It answers the question: how much does a whole AC circuit resist the current?
Precisely, impedance Z (in ohms) links the AC voltage and current amplitudes by V = I Z, the AC version of Ohm's law. A resistor's opposition R is fixed. An inductor's opposition is the reactance X_L = omega L, which grows with frequency; a capacitor's is X_C = 1 / (omega C), which falls with frequency. These combine with R not by simple addition but as Z = sqrt(R^2 + (X_L - X_C)^2), because the voltages across L and C are out of step (out of phase) with the current.
Impedance explains why a capacitor blocks DC but passes high-frequency AC, why an inductor does the reverse, and how filters and tuners select frequencies. The honest point is that impedance is more than a single number: it also carries a phase, a timing shift between voltage and current, so it is properly a two-part quantity rather than just a bigger resistance.
At a frequency where X_L = 30 ohm and X_C = 10 ohm with R = 40 ohm, the impedance is Z = sqrt(40^2 + (30 - 10)^2) = sqrt(1600 + 400) = sqrt(2000), about 45 ohm.
R = 40, X_L = 30, X_C = 10 combine to Z of about 45 ohm.
Reactance (from L and C) is not the same as resistance: a resistor turns electrical energy into heat, but an ideal inductor or capacitor stores and returns it, dissipating none — yet both still oppose the current.